(1) Field of the Invention
The present invention concerns color filters used in a liquid crystal display (LCD), and more particularly to such color filters provided with means for preventing them from being damaged.
(2) Description of Prior Art
Generally, an LCD comprises a lower TFT substrate provided with a plurality of thin film transistors (TFTs), a liquid crystal layer, and an upper substrate provided with color filters consisting of red, green and blue color filtering. Referring to FIGS. 2A to 2I, a conventional color LCD is described concerning its fabrication.
A glass substrate 1 is prepared by cutting and polishing, and then applied with a light-blocking black matrix 2 made of chromium with a thickness of 1000 to 2000 .ANG. by sputtering, to prevent the TFTs from being degraded. A negative photoresist 3 with an optimum spectral property is applied to the surface of the substrate 1 holding the black matrix. The negative photoresist may be a color photosensitive acrylic resin with a dispersion of a pigment. The photoresist is softly baked on a hot plate at a temperature 80 to 110.degree. C. for 90 seconds, and then applied with an oxygen shield film 4 of a water soluble resin for preventing the oxidation of the photoresist during exposure to light. After drying, the surface is exposed to ultraviolet rays, as indicated by the reference numeral 5 in FIG. 2D.
After the oxygen shield film 4 is subjected to deionized water (DIW) for 3 to 5 minutes to remove it, the substrate is processed by a developing agent for about 2 to 3 minutes, and then rinsed by DIW for 1 to 2 minutes, thus producing a first color filter layer 6 with an optimum red spectral property, as shown in FIG. 2F. Likewise, a second and a third color filter layer 7 and 8 respectively with an optimum green and an optimum blue spectral property are sequentially formed over the substrate 1. Of course, the second and the third color filter layer also may be made of a color photosensitive acrylic resin with a dispersion of a pigment. The first, second and third color filter layers partly overlap the black matrix 2.
The first, second and third color filter layers 6, 7 and 8, which respectively represent red(R), green(G) and blue(B), are covered by a protective layer 9 to protect and flatten them. The protective layer 9 may be produced by spreading a transparent resin such as polyimide, polyacrylate, polyurethane, etc. on the black matrix 2 and color filter layers 6, 7, 8, with a thickness of about 1 to 3 .mu.m, and then heating on a hot plate with a temperature of 150 to 220.degree. C. for 5 minutes, as shown in FIG. 2H. Finally, an indium tin oxide (ITO) electrode layer 10, which is applied with an LCD drive voltage, is formed on the whole surface of the protective layer 9 with a thickness of about 500 to 1800 .ANG., as shown in FIG. 2I, thus completing the upper substrate. Hence, there is provided a conventional LCD consisting of the glass substrate 1, black matrix 2 formed on the glass substrate 1, first to third color filter layers 6, 7, 8, protective layer 9 formed over the filter layers, and transparent electrode layer 10.
Such conventional color filters suffer the protubrances formed by foreign matter or particles in the protective layer, which cause low yield rate and cell gap-maintaining failure. Moreover, the electrical resistance and the deviation thereof increase in the regions where the black matrix contacts the substrate after forming the color filter layers, thus resulting in adverse effects on the LCD operation. Besides, the adhesive force is weakened in the regions at the boundary of the black matrix which is printed on the substrate in a sealed manner.